1. Field of the Invention
The present invention relates to a pressure roller suitable for use in a fixing device mounted in an image forming apparatus, such as an electrophotographic copying machine and an electrophotographic printer, and a fixing device including the pressure roller.
2. Description of the Related Art
As fixing devices mounted in electrophotographic printers or electrophotographic copying machines, there has been known a heat roller-type fixing device including a halogen heater, a fixing roller heated by the halogen heater, and a pressure roller brought into contact with the fixing roller to form a nip portion.
Additionally, there has been known a film heating-type fixing device including a heater having a heat generating resistor formed on a substrate made of ceramics, a fixing film moving on the heater while being brought into contact with the heater, and a pressure roller forming a nip portion together with the heater via the fixing film.
Each of the heat roller-type fixing device and the film heating-type fixing device is configured to heat and fix a toner image onto a recording material carrying an unfixed toner image thereon while the recording material is pinched and transported at the nip portion.
A releasing layer is generally provided on a surface layer of the fixing roller or fixing film, and a surface layer of the pressure roller, which are used in these types, to prevent the toner from adhering thereto. A fluorine resin can be used as the releasing layer.
However, since the fluorine resin is a high electrical insulation material, the fluorine resin has properties in which it is easily electrically charged and static electricity is hardly escaped therefrom. For this reason, if the recording material with the unfixed toner image is transported to the nip portion of the fixing device, an electrostatic offset image (hereinafter, referred to as electrostatic offset) is likely to occur, in which, the unfixed toner electrically adheres to a surface of the fixing roller or fixing film and is then fixed to the recording material when the fixing roller or fixing film revolves.
Accordingly, a method of dispersing a charge control agent into the fluorine resin, a method of decreasing resistivity of a portion of a layer or a plurality of layers of the pressure roller, and a method of adding an antistatic agent into a rubber layer of a fixing roller or a fixing belt have been proposed to prevent the fluorine resin of the releasing layer from being electrically charged.
For example, Japanese Patent Application Laid-Open No. 04-19687 discusses a fixing device including a fixing member having a heat generating element therein, and a pressure roller placed opposite to the fixing member in a freely rotating manner, in which the pressure roller has an electrically conductive core metal, an elastic layer formed on the core metal, and a surface layer of an electrically conductive PFA tube formed on the elastic layer.
In addition, Japanese Patent Application Laid-Open 2002-258649 discusses a pressure roller which includes a releasing layer, an adhesive layer, an elastic layer, and a core metal, which are formed in order from the surface of the pressure roller, wherein the releasing layer has an electrical resistance property, the adhesive layer has an electrical conductive property, and carbon black is used as particles dispersed in the adhesive layer.
Furthermore, Japanese Patent Application Laid-Open 2006-265340 discusses a nonconductive silicone rubber composition for a fixing roller or a fixing belt, in which 0.001 to 2 parts by mass of antistatic agent is contained in 100 parts by mass of an organopolysiloxane containing at least two alkenyl groups bonding to at least two silicon atoms in one molecule and in an effective amount of curing agent for curing the organopolysiloxane, and in which the composition is applied to a rubber elastic layer.
However, the configurations of the related art have the following problems from the viewpoint of electrostatic offset and toner stain.
When a surface potential of the pressure roller is excessively increased by frictional electrification when an electric field to attract the toner to the surface of the fixing film or fixing roller from the recording material is generated, and thus the electrostatic offset occurs on the recording material. Therefore, an offset image is continuously produced on the whole image. Meanwhile, the toner stain is the one in which the offset toner adheres to and accumulates on the surface layer of the pressure roller. A lump of toner adheres to the underside of the recording material at any timing, which causes an image defect.
In the case of the related art where the conductive PFA tube is provided as the surface layer of the pressure roller, the toner stain easily develops on the pressure roller. The conductive PFA tube is made by adding carbon to insulating PFA to produce conductivity. As compared with the insulating PFA tube with no conductive material, its electrostatic offset is superior, while its releasing property of the toner is inferior.
If the content of the carbon is reduced, the releasing property is improved, but the electrostatic offset is deteriorated. Accordingly, in the conductive PFA tube, the electrostatic offset and the stain of the pressure roller are in a trade-off relation with respect to the addition of the carbon.
In addition, in the pressure roller in which the releasing layer of the pressure roller is made of only by a fluorine resin tube of an electrical insulating property and the adhesive layer between the releasing layer and the elastic layer contains electronically conductive particles such as carbon black dispersed therein, the ability of suppressing the electrostatic offset may be lowered depending upon the configuration of the fixing device.